The present invention is directed to a method for removing high contents of mercury and water from fluids and eliminating the generation of mercury-containing disposal wastes. More particularly, this invention relates to a method of simultaneously removing high contents of mercury and water from natural gases, which allows for regeneration of the adsorbent, recovery of the captured mercury and direct use of the regeneration gas as fuel.
It is often desirable to simultaneously remove water and mercury from fluids, such as from hydrocarbon fluids, an example being natural gas. Simultaneous removal processes are described in U.S. Pat. No. 4,892,567 which is herein incorporated by reference. This patent describes a molecular sieve which is modified with gold or silver for the purpose of simultaneously removing mercury and water from fluids. Molecular sieves are disclosed in this reference, in particular zeolite A, as being useful for removing mercury and water and for being capable of being regenerated. This reference discloses the use of gold or silver in amounts of 0.001-15% by weight as a coating on or impregnation in the zeolite. No direct suggestion is given, however, for removing massive quantities of mercury from the fluid while providing for a regenerative process which allows for the recovery of useful mercury and low mercury-content regenerative gas useful as fuel.
Conventional methods of removing high amounts of mercury generally involve using sulfur on carbon mercury removal bed (S/C HGR) to remove the major amount of mercury. The disadvantage of this method, however, is that the spent adsorbent often contains up to 10% or more mercury and is not able to be regenerated. The used adsorbent must then be disposed of as waste, creating environmental concerns. Attempts to transport mercury wastes raise issues regarding regulations governing transportation of hazardous waste materials. In addition, conventional methods of removing high amounts of mercury from fluids usually involve a secondary mercury removal step prior to liquefaction. Conventional methods are often therefore inefficient and costly.
While U.S. Pat. No. 4,892,567 has disclosed a means for overcoming some of the disadvantages of the conventional mercury removal methods, it has not focused on regeneration whereby high amounts of mercury are substantially recoverable and the regeneration gas is useful as fuel. Thus, the present invention is an improvement over the teachings and disclosure of this patent.
Water is effectively removed from hydrocarbon fluids, such as natural gases, by means of molecular sieves, particularly the synthetic crystalline zeolite known as zeolite A.
Zeolite A contains cavities formed by sodalite cages stacked in simple cubic form. The sodalite cages are made up of a truncated octahedral having a silica or alumina tetrahedron at each point. The cavities are surrounded by eight oxygen atoms, and are partially blocked by cations that balance the charge on the oxygen atoms. In zeolite A, each alumina moiety is balanced by two positive charges. If the cation is sodium, the cavity is reduced to about 4.2 angstroms in diameter. If the cation is potassium, the cavity is reduced to about 3 angstroms in diameter. If the cation is calcium, the cavity is reduced to about 5 angstroms in diameter.
Zeolite A having sodium, potassium and calcium ions is known as zeolite 4A, zeolite 3A and zeolite 5A, respectively. The pore diameters of zeolite A make them especially suitable as drying agents, since the pores are large enough to accommodate water molecules, but not most other molecules found in nature. Zeolite A is further described in U.S. Pat. Nos. 2,882,243; 2,982,612; and 3,650,687. When a zeolite used for drying fluids becomes saturated with water, it must be regenerated, which is often accomplished by heating with flowing hot gas. Zeolite 4A is the most commonly used molecular sieve for drying natural gas.
In the inventive regenerative mercury removal process, it is preferred that the capacity for mercury removal is large enough so that all mercury contained in the fluid can be removed in the period during which the adsorbent becomes fully saturated with water and must be regenerated.
Natural gas may contain as much as 250 ppb (micrograms/m.sup.3) mercury. Natural gas is, in many commercial liquefaction operations, transported to and cooled in aluminum heat exchangers. Mercury present in the natural gas causes corrosion of the aluminum and must be removed. Additionally, high contents of mercury cannot remain in the regenerative gas if it is to be used directly as fuel.
There are a number of methods for removing mercury from fluids such as gases. U.S. Pat. Nos. 4,101,631 and 4,474,896 describe the removal of mercury from gas streams by means of sulfur or sulfur compounds on supports such as zeolite an activated carbon. These methods are capable of reducing the level of mercury to about 0.1 ppb. Even this level of mercury in a stream, however, can injure aluminum heat exchangers.
There is a need to reduce the level of mercury in fluids to below 0.01 ppb or less. In order to be commercially feasible, the method must be inexpensive as well as efficient. It would be especially desirable to remove mercury and water simultaneously and repeatedly from a fluid with the same agent, so that, following contact with the agent, the level of mercury is less than 0.01 ppb and the level of water is less than 1 ppm. It is even further desirable, however, to find an effective and efficient means of removing high amounts of mercury from a fluid in a simplified one-step process, regenerating the adsorbent and recovering the mercury, thereby eliminating the need for disposal of mercury-containing adsorbent waste.
It is a principal object of the present invention to provide a means for removing and recovering high amounts of mercury from a fluid and providing a means for regenerating the adsorbent many times. A further object of the present invention is to simultaneously remove high amounts of mercury and water and regenerating the mercury such that the resultant regeneration gas can be used directly as fuel. The additional objective is to remove the mercury from the atmosphere and to recover the mercury as a valuable product. These and other objectives will become apparent to those of ordinary skill in the art.